Reduction of distortion in multiplex repeaters



Patented Jan. .26,- 1926.

UNITED' STATES PATENT [OFFICE:

HARRY NYQUISL'OF ELMHURST, NEW YORK, ASSIGNOB TO AMRICAN TELEPHONE .ANDTELEGRAPH COMPANY, A. CORPORATION F NEW YORK. l

nnnUc'rIoN or Drs'ron'rIoN 1n n'UL'rrPLnx nnrnn'rnns.

Appncationmea Haren 20.1922. serial no. 545,29a Y To all 'u1/tomI t mayclmcem."

Be it known that I; HARRY NrQurs'r, residing at Elmhurst, in the countyof Queens and State of' New York,have invented certain Improvements intheReduction of Distortion in Multiplex Repeaters, of which thefollowing is a specification.

This invention relates to multiplex signaling, and more .particularly tomeans for reducing the eii'ect of distortion, in repeaters or othertranslating devices used in connection with such a system.

Heretofore, in the operation of multiplex vcarrier systems, it`has beenfound that the repeater tubes employed at points along the line fortransmitting carrier frequencies produce certain undesirable effectswhere the same tube is used to transmit the frequencies of a number ofVchannels. This is for-the reason that the tubes do not have a straightline characteristic, andhence produce a certain amount of distortionwhich causes what might he termed mutual crowding. of the channels. 'Bythis is meant that Where one frequency exists in -a repeater or' otherform of translating circuit along with other 'frequencies,.itdecreasesthe amplitude of all of the other frequencies by a certainamount, as compared Withthe amplitudes the frequencies would have ifeach were'on the circuit alone. This eiiect at the receiving end of Ythecircuit is very similar to the effect Y which would be produced by theintroduc- 'tion of a loss in' the circuit by any other means, such', forexample, as a resistance shunted acrossv the circuit. AThis eiiect isvery undesirable in a carrier telegraph sys- Vtem involving a largenumber of channels employing carrier frequencies 'within the usual voicerange, for 1n such a system 1t is almost essential to economic'workingof the system that the channels be narrowand closely spaced, vso thatall the channels may be amplified bythe same repeater tube.

i In accordance with the present invention, it is proposed to overcomethis difficulty by so arranging each carrier channel that when thecarrier frequency is applied to the channel, the normal impedance of thecircuit will be reduced by an amount" suiiicient to compensate for thereduction inl amplitude to which the frequencies then existing on thecircuit would be subjected.

The invention may now be more fully understood by reference to thefollowing description, when read in connection with the accompanyingdrawing, Figure 1 of which illustrates in simplified form the circuitarrangement of a. carrier system embodying the present invention, andFig. 2 of which shows a number of curves illustrating the principles ofthe invention.

Referring to Fig. 1, Le and Lw designate two transmision lines forming afour-wire .circuit for the transmission of carrier frelective device orlilteryRFs, a detector and a receiving relav RR.

The filter RS may be of the Well known Campbell type and is illustratedas consistyingA of two sections, each having a series resonant elementand a shunt anti-resonant element. The filter at the side adjacent thebus-bars terminates in a full series element in order to present thegreatest impedance to frequencies lying Without the range of freetransmission. The detector l) maybe v ofy any well known type, such, forexample, as a vacuum tube detector. It will be understood that-each ofthe receiving channels includes apparatus similar to that illus.-

'trated in connection with the channel RLS,

and that the lters'of each channel will be so designed as to freelytransmit a range of frequencies differing from the other channels.

The .receiving relay RR is arranged .to A transmit vthe direct currentimpulses detected by the detector D toY an ordinary telegraph lineL,Ywhich is balanced by means of anetwork N, the transmitted impulsesbeingi' connected tors. neutral point between the line L and the networkN in order not to operate the transmittingrelay TR associated with thetelegraph line L.

' The input side of therepeater Rw is connected to bus-bars 11, fromwhich transmit- .ting channels TL TL TLS, TL TL5,etc., l

are branched. Each channel, such as the channel TLB, includes a filterTFv which may be of thegwell known Campbell type. As illustrated, it,like the filters associated with the receiving channels, consists of'two sectionseach comprising a series resonant element and al shuntyanti-resonant element.

The section adjacent the bus-bars terminates .mitting carrieroscillations. `Thesource S3 for the channel TL3 is associated therewiththrough a transformer 12, thesecondary circuit of which is included inthe channel TL3 and may be opened or closed under the control of thearmature 13 of the transmitting relay TR. Direct current impulses fromthe line L will, therefore, be translated into groups of carrieroscillatioris from the source 3, which may be transmitted through thechannel TL3 to the line Lw. The transmit- I ting apparatus-associatedwith each channel v fas is illustrated -at 5-6-7 i form indicated at202122 andas seen,

will be similar to that illustrated in connection with the channel TLB;f l

Before describing the arrangements of the invention designed toovercome-the condition known as crowding, it will be explained "howthi-s condition arises in connection with a repeatenfor example.Referring to Fig.

i 2, 1-2-'3 is a curve illustrating the characteristic of a vacuum tuberepeater. It will be noted that the curve is nota straight lme and kthatits angle with the axis 2-4 becomes less as the curve recedes from saidAaxis. They condition represented by the curve 1--2-3 is, of course,exaggerated in 45, order to morel clearly show what takes place,

and in practice it will b understood that the characteristic curve of avacuum tube repeater" will more nearly approach a .straight line.However the phenomenon known asv crowdin will exist, though to a lesserdegree than lllustrated in connection Let us suppose a'simple sine .wavesuch and having a definite frequency and constant amplitude is impressedupon the repeater. The waveI appearing in the output circuit will takethe 7 the amplitude of the wave in `the output .circuit will beproportional to that of the input wave and the frequency will be thesame. For theI sake of simplicity, lthe am- .'plitude of the output waveis made equal to' that'of the input wave which would co'rrespond to acondition in wh1c h no actual .amplification takes place. Thls, however,

has no effect upon the principle about to be explained. y

' Assume now that in addition to the wave having the same frequency andamplitude as the wave '5-6-7, another wave of the same amplitude but ofdifferent frequency be impressed upon the amplifier. The two.v

waves now impressed upon the amplifier will take the form indicated at23-24. The two waves, when combined in the same circuit, however,produce a combined wave having the form illustrated at 7-25-26. Owing tothe non-linear characteristic illustrated at 1-2-3, the Goutput wavewill take the form 22f 27-4. This wave, it will be noted, issubstantially of the form of the wave 7-25-26, but its amplitude is lessthan that of the wave 7-25-26. If the wave v22---.274 be resolved intoits components,

it will 'be foundto comprise two waves 28 and 29, corresponding infrequency to the waves 23 and 24, respectively, but each Vbeing ofsmaller amplitude than the original waves impressed upon the amplifier.Thus it will be seen that while the wave 24, if impressed upon theamplifier alone will produce a wave of the same amplitude -in the outputcircuit, when another wave, such as 23, Iis impressed upon theamplifierat the same time, the wave 29 in the output circuitcorresponding to the applied Wave 24, will have an amplitude less thanit would have vif the wave 23 were not applied.

In order to overcome this diiiiculty, a condenser 15 is included in thetransmitting ychannel TL and a high impedance alternating current relay16 is shunted about the condenser. The contacts of the relayv 16 controla resistance 17 shunted across the channel TLsin parallel with the sideof the filter TF3 adjacent the bus-bars 11.. Whenever the carrierfrequency is applied to the channehthe relay 16 disconnect-s' theresistance 17 shunted across the channel, and

'when the carrier frequency is no longer applied to the circuit, 'theshunt resistance 17 is again bridged across the channel. The condenser.15 acts as a shunt for the high impedance relay 16, Vso that theinclusion 0f this relay in Ithe circuit does not materially affect theimpedance ofthe lter as viewed from the source of carrier oscillationsS3. An arrangement, similar to that just described in connectionwiththe'channel TL,x is associated channels. l .i l .A

If we consider the action of the transmitting channels on the assumptionthat the shunt -resistances, Such'as 17, are notv providedgit `will beseen that when a frequency withV each of the transmitting is applied' toa single onev of the channels,

the limpedances of the filters in the other channels will be so largethat 'very little of the energy transmitted from-the active channel willbe diverted from the lineL, into l comes active and a carrier frequencyisthe other transmitting channels. When, however, shunt resistances suchas 17 A are provided, as illustrated, and but one channel is act-ive, acertain percentage of the energy from the Achannel vwill be divertedfrom the line Lw into each of the other.

channels, owing to the reduced impedance of the channel due to shuntingthe resistance 17 across the terminal of the filter. This reduces theamplitude of the carrier frequency supplied from the' active channel tothe linel Lw by an amount which may be made by proper proportioning ofthe resistance 17 justequal to the reduction in amplitude due tocrowding.

Consequently, when a second channel betransmitted therethrough theopening of the shunt 'through the resistance 1n the second channelincreases the impedance of the second channel to the frequencytransmitted from the first active channel, so that agreater proportionof the energy vfrom .the first channel is transmitted to theline L. The

increased amplitude -of the current transmitted to the input side of therepeater in y the line will, if the shunt resistance is properlyproportioned, be sutlicient to make up for the reductionin* the outputof the repeater due to crowding, and consequently, the amplitude of thefrequency from the Y iirst channel appearing in the output circuit fromthe spirit of the invention as de of the repeater will be substantiallythe same whether any other channel isv active or not.l It will be atonce apparent that any desired combination of channels' may betransmitting without substantial variation in the amplitude of thefrequency transmitted froml each channel as it a pears at the receivingend. The reduction 1n amplitude which this which a plurality offrequencies lfrom a pln-` rality of channels may be transmitted over acircuit including. a common repeater, the

method of overcoming the eiects ofcrowding of the channels in therepeater when more than one frequency is transmitted at the same time,which consists in increasing the transmission of the circuit for thefre-` quencies of the other channels lwhen a given ft "the energy thusdiverted into the impedance associated with an inactive channel when itbecomes active.

channel i's l transmitting. ff

.'2. In a multiplex signaling whicha plurality of frequencies fromafpliil rality f channels may" be transmitted through a common repeater,the method of overcoming the effects of crowding of the V*channels inthe repeater when more than one frequency. is transmittedV at the sametime, which consists in diverting a certain amount `of the energytransmitted by a given channel for each of the other channels which arenot then active, and reducing the amount of energy thus diverted for anyof said other `channels when they become active.

3. In a multiplex signaling system in which a plurality of frequenciesfrom a plurality of channels may be transmitted through a commonrepeater, the methodY of overcoming the edects of crowding of thechannels in. the repeater when more than one frequency is transmitted atthe same time, which consists in diverting a certain amount of theenergy from a channel which is actively transmitting into each of theinf active channels, and increasing the imped-v ance of the inactivechannels to frequencles outside the range of the particular mactivechannel when the inactive channel actively transmits.

,4. In a multiplex 'signaling system in which Aa plurality of channelsmaybe transmitted through a common repeater, the method'of overcomingthe eiectsV of crowding of the channels in the repeater when I more thanone frequency is transmitted at the same time, which consists indiverting from the repeater a certain amount of the -energy from achannel4 which isactively' transmitting, controlling the amount divertedby impedances in shunt with the other channels, and reducing the amountthus diverted as other channels become active.

5. In a signaling system, a transmission line including a repeater, aplurality of 'channels transmitting at diiferent frequencies throughsaid repeater, means operating Awhen a given channel is activelytransmitting to divert from the repeater ahportion of the energy `fromthe transmitting channel, and means controlled bythe other channels asthey become active in transmitting through said repeater, terminalapparatus 1 for the transmittingV channels, said terminal apparatusincluding 'means operating when 'a given channel is transmitting topartially4 divertthe energy from said channel into an for reducing theamount of energy which is impedance associated with the, inactive jransmitting channels, and means to reduce 7. In. a signaling system,- atransmission line, a repeater therefor, a plurality of transmittingchannels for transmitting through said repeaterat different frequencies,and means controlled by each channel for increasing the transmission ofthe repeater circuit for frequencies of the other channels when saidchannel is transmitting.

8. In a signaling system, a transmission line, a-repeater therefor, aplurality of transmitting` channels for transmitting through saidrepeater at different frequencies, and means associated with eachchannel for controlling an impedance associated with 'that channel asregards frequencies transmitted by the other' channels when that channelistransmitting.

9. In a1 signaling system, a transmission line,.a repeater therefor, aplurality of terminal transmitting,` channels connected in multiple totransmit through said repeater,

the im edance of the channels being such that when any channel istransmitting a portion of its energy will be diverted from the'lineto'the parallel channels, and means associated with each channel .todecrease the amount of energy of other channels diverted thereto`When'the particular channel is transm'tting v 0; In a multiplexsignaling system involving transmission loss due toV mutual w crowdingof the channels, means to produce a variable transmission loss, andmeans for y varying the transmission loss Jproduced by said means in amanner complemental to the variation in transmission loss dueto crowd-March, 1922. HARRY` NYQUIST.

